2102 Inorganic Chemistry, Vol. 15, No. 9, 1976
Pietropaolo et al. Contribution from the Istituto di Chimica Generale ed Inorganica dell’Universita, 98 100 Messina, Italy
Displacement of Dienes from Planar Complexes. 1. Reaction of (1,5-Cyclooctadiene)dichloropalladium(II) with 2,2’-Bipyridyl E. R O T O N D O , G. T R E S O L D I , F. FARAONE, and R. P I E T R O P A O L O * Receiued October 31, I975
AIC507880
A kinetic study on the reaction between (1,5-cyclooctadiene)dichloropailadium(II) and 2,2’-bipyridyl (bpy) in methanol, leading to Pd(bpy)Clz, is reported. The observed kinetic law of the process i s
which suggests that the chloride ions are first substituted and that the dipositive complex [Pd(COD)(bpy12+ ( C O D = 1,5-cyclooctadiene) is formed as an intermediate. The stage of the reaction leading from [ P d ( C o D ) ( b ~ y ) ] ~to+ Pd(bpy)Cl2 was independently investigated following the reaction of [Pd(CsHlzOCH3)(bpy)]+ with H+ and CI- in methanol. The experimental results are interpreted considering the formation, during the course of this reaction. of unreactive five-coordinated [Pd(COD)(bpy)CI]+ in equilibrium with the reactive four-coordinated species [Pd(COD)(bpy)]*+,
Introduction Ligand substitution reactions in coordination compounds occur with a variety of mechanisms, depending on the nature of the reagents, the solvent, and the coordination number. The behavior of d8 transition metal complexes such as those of RhI, Pt”, Pd“, and AulI[ appears to be simple, in the sense that the substrates generally undergo a bimolecular substitution with transition states in which both the entering and the leaving groups are bonded to the metal. Kinetic studies of the displacement of polydentate ligands from ds transition metal compounds have also been reported in the recent literature.’-6 Generally the mechanism involves opening of the cheiate ring, followed by competition between ring closure and displacement of the ligand. Therefore simple kinetic expressions are not common even if any single mechanistic path must obviously be consistent with those known for substitutions of unidentate group^.^ In this context it was interesting to investigate the displacement of some diolefins from palladium(I1) compounds such as Pd(diene)Cla, (diene = 1,5-cyclooctadiene, dicyclopentadiene, norbornadiene). In the wake of some of our previous r e s ~ l t swe ~ , anticipate ~ that the presence of a strong trans-labilizing ligand in the complex, such as a diolefin, should allow the chloride ions to undergo an easier substitution. The aim of the work is then to investigate the effect of the charge of the complex on the stability of metal-diolefin bonds in palladium(I1) compounds and the susceptibility of the coordinated dienes to undergo nucleophilic attack as related to the structure of the diolefin and the formal charge of the metal. This paper reports a kinetic study of the reaction Pd(COD)Cl,
+ bpy
+
Pd@py)Cl,
+ COD
(COD = 1,5-cyclooctadiene; bpy = 2,2’-bipyridyl) in 95% aqueous methanol. Results related to other diene complexes will be described in forthcoming papers. Experimental Section Preparation of Compounds. Pd(COD)C12I0 and Pd(bpy)Clzl‘ were prepared as reported in the literature. Pd(bpy)Brz was prepared by treating KlPdBr4’2 with 2,2’-bipyridyl in methanol. Pd(bpy)Xz complexes (X = CI, Br) were also obtained by treating Pd(COD)X2 with 2,2’-bipyridyl or [Pd(C~Hl20CH3)(bpy)]PF6with H X under the same kinetic conditions. (5-Methoxycyclooctenyl)(hipyridyl)palladium(II) HexafluoroA 142-mg sample of phosphate, [Pd(CaH120CH3)(bpy)]PFs. Pd(COD)C12 (0.5 mmol) in CH3OH was treated, under stirring, with
169 mg of A g N 0 3 (1 mmol). AgCl was filtered off and 78 mg of 2,2’-bipyridyl was added to the yellow solution. After addition of excess of NH4PF6. white needles slowly precipitated and were washed several times with C H 3 O H . The ir spectrum of this compound shows a very strong band at 1070 cm-’ due to the C-O-CH3 grouping and other bands respectively at 1595, 1605. and 765 cm-I due to the coordinated bi~yridy1.I~ A broad band centered at 830 cm-I can be attributed to the PF6- anion.14 The value of Ah1 for a 5 X M acetone solution of this salt is 150 Q-l cm2 mol-I thus confirming that the compound is a uni-univalent electrolyte. Anal. Calcd for PdC19H230N2PF6: C, 41.73; H, 4.2; N , 5.12. Found: C, 41.5: H , 4.1; N , 5.2. (Cyclooctadiene)(hipyridyI)palladium(II) Bisperchlorate, [Pd(CsH12)(hpy)](ClO4)2. A suspension of 70 mg of [Pd(C8H120CH,)(bpy)]ClO4 was treated with an excess of HC104 in CH2C12. The green-yellow product formed was washed several times with acetone. Its ir spectrum shows a broad band centered at 1080 cm-I due to the C104- anion whereas other bands a t 1600 and 770 cm-l can be attributed to the presence of bipyridyl. The value of AM for a 2.49 X M acetone solution of this salt is 294 Q-I cm2 mol-’ thus confirming that the complex is a bi-univalent electrolyte. Anal. Calcd for PdClsH2oN2ClzOs: C, 37.95; H, 3.53: 0. 22.46; N, 4.91. Found: C . 38.04, H, 3.64; 0, 22.43; N , 5.05. Kinetics. Separate solutions of the complex and reagents were prepared in methanol containing 5% by volume of water. The concentrations of chloride or bromide and proton ions were determined by standard titrimetric methods. The ionic strength was maintained constant at 0.5 M. The reagent solutions were separately brought to reaction temperature and then mixed in the thermostated cell of an O P T I C A C F 4 R double-beam recording spectrophotometer; the kinetics were followed by observing absorption changes in the ultraviolet region of the spectrum. Any single kinetic run v;as carried out with concentrations of halide and H+ large enough to provide pseudo-first-order conditions. The concentration of palladium(I1) complexes, in the reaction mixture, was in the range 1 X 1V4-2.5 X M. Fresh solutions of the complexes were always used. Pseudo-first-order rate constants, kobJd (s-’), were calculated from slopes of linear plots of In ( A , - A t ) vs. time ( A is the optical density).
Results and Discussion The reaction Pd(COD)Cl,
-
+ 2,2’-bpy H’,
C1-
Pd(bpy)Cl,
+ COD
(1)
proceeds rapidly and smoothly to completion in 95% aqueous methanol a t 25 O C in the presence of hydrochloric acid and lithium chloride. Preliminary results suggested that the rate of olefin displacement by C1- from Pd(COD)C12 is very slow and does not affect the reaction of the same complex with 2,2’-bipyridyl.
Inorganic Chemistry, Vol. 15, No. 5, 1576 2103
Displacement of Dienes from Planar Complexes I
I
/
= I
103 [E(..].
M
Figure 1. Plot of kobsd (s-') values against the analytical concentration of bpy a t [Cl-] = 0.5 M and variable hydrogen ion concentratlOnS: A, [H+]= 4.94 X M ;0, [H'] = 10" M;@, [H'] = 3.05 X lo-' M.
The course of reaction 1 was followed at X 350 nm and only one stage was detected. At lower wavelengths it was impossible to follow any kinetics because of the large absorption of bipyridyl. However, when Pd(COD)C12 and bpy were allowed to react in a 1:l ratio, Pd(bpy)Clz was detected, as final product, from two characteristic bands between 300 and 3 10 nm. Kinetic runs were carried out at seven different chloride concentrations and at three hydrogen ion concentrations. Within each set of runs [Cl-] and [H+] were kept constant as the analytical concentration of bipyridyl was changed. Pseudo-first-order conditions, with respect to the bipyridyl, were provided by adding a large excess of ligand. In acid media the main species is bpyH+. We found, in fact, that the absorbance and the shape of the ultraviolet spectrum of a methanol solution containing a loe4 M concentration of bipyridyl do not change, between 400 and 220 nm, when the hydrogen ion concentration ranges from to 5 X lo-* M. The absorption maxima (-240 and -300 nm) and the extinction coefficient at 300 nm (1.65 X lo4) are also very close to those observed for aqueous solution of bpyH+.IS The equilibrium to be accounted for in the experimental conditions used is bpyH+ $ bpy
+ H+
(2)
which lies far to the left. Although in all cases the concentration of free 2,2'-bipyridyl was smaller than that of the substrate, it was kept constant all during the course of the reaction by the rapid acid equilibrium (2). In the absence of pK, values for the base employed in aqueous methanol, we have been forced to use the pKa value of 4.43, determined in aqueous solutions.I6 This assumption appears to be quite reasonable since it has been found, on the basis of potentiometric determinations, that the basic strength of amines in water is a reliable index of their base strength in aprotic s01vents.I~ This should be also true for methanol, which has a greater similarity to water than to nonprotolytic solvents. Results reported in the recent literature show that the reactivity of bipyridyl solutions toward transition metal complexes could be complicated because both free (bpy) and protonated (bpyH+) species are potential reagents.' 1,18 Therefore we tried, first of all, to establish if one or both of the species involved in equilibrium 2 were reagents in reaction 1. Each set of values of the observed rate constants at [Cl-] = 0.5 M and variable [H+], at a given proton concentration,
Figure 2. Linear dependence of the slopes of the plots of kobsd (s-l) values against the analytical concentration of bpy, Y , on I / [H+]at [Cl-] = 0.5 M.
Scheme1
\
J
L
I
.........
J
, I1 -
-
Pd(BiPu)Cl,
L
gives good straight lines when plotted against the analytical concentration of 2,2'-bipyridyl as shown in Figure 1. The slopes of these plots, r, exhibit a linear dependence on the reciprocal of [H+] with zero intercept (Figure 2). On the basis of these findings and according to equilibrium 2 we conclude that bpyH+ is not a reactive species. Experimental rate constants at [H+] = 0.5 M and variable [Cl-] show a different behavior when plotted against the concentration of free bipyridyl. At higher [Cl-] no intercept was observed, whereas at lower [Cl-] well-defined intercepts can be detected (Figure 3). Furthermore gradients of all plots, s, depend on the chloride concentration and their inverse, l/s, exhibit the linear dependence with a definite intercept, shown in Figure 4, when reported against [Cl-1. These results indicate an overall rate law of the form (3)
where d is a term dependent on [Cl-1. A reasonable mechanism accounting for these results is shown in Scheme I. Since only one stage can be detected at X 350 nm, it may reasonably be assumed either that the step leading from the intermediate IV to Pd(bpy)Clz is a fast one or alternatively that, at the wavelength used, no absorption change accompanies this reaction. In any case we can discuss our results as referring to the formation of the intermediate IV, and the
2104 Inorganic Chemistry, Vol. 15, No. 9, 1976
Pietropaolo et ai.
a 0 3
32
I
28
Figure 3. Plot of kobsd ( s - I ) values against the concentration of free bipyridyl at [H+]= 0.5 M and variable chloride ion concentrations: e , [ci-]= 5.4 x 10-3 M;B, [ci-]= 10.5 x 10-3 M ; e , [a-1= 22 x 10-3 M; O, [ c i q = 100 x 10-3 M ; 0 , [a-] = 208 x 10-3 M;x , [c17 = 315 x 10-3 M; ., [ci-1=425 x 10-3 M.
Equation 4 reduces to the observed rate law of eq 3 where a = K k l , b = K , c = 1, and d = Kk2/(K [Cl-1). Using a computer program we found that the best fit of eq 4 was obtained with the average values of kl = 6.80 X lo4 f 6.7 X lo3 M-‘ s-l, K = 3.74 X f 5.5 X and k2 = 5.55 x 10-2 f 3.49 x 10-3 s - l . Because of the higher trans effect of the 1,5-cyclooctadiene in comparison to that of the chloride, it can be considered that in Pd(COD)C12 the chloride ions are the most labile.8~9The mechanism in Scheme I involves an extensive solvation of the diene complex I leading to the intermediate 11. This undergoes a bimolecular substitution of the solvent by 2,2’-bipyridyl, followed by a fast chelation in the cis position to give [Pd(COD)(bpy)I2+. This intermediate further reacts and Pd(bpy)Cl2 is formed as final product. The term, independent of [bpy], represents the route of the reaction through intermediate 111. A second-order kinetic pattern, relative to the kl term, results from the solvolysis of the palladium substrate being faster than the subsequent reaction of the solvated species with the entering nucleophile. This is a t variance with the customary behavior of d8 complexes toward nucleophilic displacements, where solvolysis is the rate-determining step of the solvent-assisted reaction p a t h ~ a y .However ~ this may be due to the fact that bpy is a poor reagent.” Although we did not observe any reactivity of the species Pd(COD)C12 toward bipyridyl, we cannot exclude its occurrence. We should think that the extensive solvation of the starting complex coupled with the high reactivity of the in-
+
100
2m
3w
400
IO3 [Ci-]
Y
Figure 4. Dependence of the reverse of the slopes of the plots of kobsd (s-’) values against the concentration of free bipyridyl, 11s. on [Cl-] at [H+]= 0.5 M.
derived rate law, at lower [Cl-] if k-z[Cl-] the form
